Difference between PR and CUF

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Monitoring is the key in any business and when it comes to solar power generation, monitoring becomes one of the most important task. There are several parameters that one needs to measure and monitor on daily basis to evaluate the health of a solar power plant. One of the major parameter is Performance Ratio (PR). A PR gives a good idea of a performance of a plant, with respect to the solar irradiation, it’s basically a parameter which gives ratio of input vs output. Generation is the output and solar irradiation is the input. A typical formula to measure the PR is as following -

PR = Generation*100/(Solar Irradiation*plant capacity) %

Above formula would give you a good idea about the performance of a plant at any instance during the day. Any value above 80% is considered as good.

PR is useful in most of the cases, but there’s another term which is used extensively, though not a very good indicator of the plant’s performance in terms of input v/s output, but it’s a useful indicator for the management, who would like to know a plant’s performance in terms of it’s capacity and hence, the expected generation. It’s called Capacity Utilization Factor or CUF. A general formula for CUF is

CUF = (generation/capacity*24)*100 %

The 24 in the above formula indicates 24 hours of a day. CUF is calculated for the whole 24 hours, though a solar plant operates for a maximum of 12 hours in a day. But CUF gives a good idea about a plant’s generation performance, in respect of it’s total capacity, it doesn’t consider the weather conditions. A healthy CUF value for a plant is considered to be between 16 to 20 %.

Major difference between PR and CUF is the inclusion of solar irradiation in the PR calculation and leaving it in the CUF formula.

To understand that how CUF can be used to hide the poor performance of a plant, let’s consider one of the scenario when solar irradiation is better than the average for any particular day. On that day, if the plant is underperforming, the generation, which should be better than the average day due to the increase in solar irradiation, is not increased according to the solar irradiation. This fact will not be highlighted if we calculate the CUF, because the CUF will be based on the generation only and it would be similar to the average day. But if we calculate PR for the same day, it would be below the average mark, because the generation hasn’t increased in accordance to the irradiation. So, PR is a good choice of parameter to evaluate the performance of a solar plant, but CUF, not so much.